Recent advances in molecular genetics make it feasible to consider genetic testing of the fetus via the small numbers of fetal erythroid cells that migrate across the placenta into the maternal circulation during pregnancy. Such a maternal blood test could ultimately complement, or perhaps even replace such invasive procedures as amniocentesis and chorionic villus sampling. It is critical to the development of these molecular techniques to first definitively identify and distinguish fetal cells from maternal cells, and second determine the gestational variability in the occurrence of fetal erythroid cells, including nucleated erythroblasts, in the maternal circulation. We have developed a flow cytometric method of unambiguously and directly identifying, quantifying and isolating fetal cells from maternal peripheral blood samples. Our technique takes advantage of the fact that the cell surface erythroid lineage marker glycophorin A is also the genetic determinant of the polymorphic MN blood group, and utilizes allele-specific monoclonal antibodies to the M and N forms of the glycophorin A protein. By double labeling with these two antibodies and using two-color flow cytometry, we can unambiguously distinguish fetal cells in maternal blood in those 50 percent of cases where the mother and child are genetically discordant at this locus. In pilot studies we have determined that there are detectable fetal erythroid cells in the maternal circulation prior to chorionic villus sampling (9-12 weeks gestation) and after full-term uncomplicated birth, and that there is an increase in fetal cell frequency over this time. We propose to apply these techniques to a population of 100 informative maternal/fetal pairs identified following normal delivery with samples banked throughout gestation. These techniques will then be applied to incident maternal/fetal pairs with a variety of adverse outcomes, including pre- eclampsia. This study will also provide samples for the development and application of techniques for fetal cell enrichment and direct sorting of propidium iodide-positive fetal cells (reticulocytes and nucleated erythroblasts). Putative fetal cells will be captured as a population in a test tube, or laid out on a slide grid for direct microscopic analysis for nucleated cells. The fetal derivation of sorted reticulocytes and nucleated cells will be verified using allele-specific PCR for glycophorin A mRNA and by PCR and/or FISH analysis for the presence of a Y chromosome in male conceptuses.